Thanks to improvements in energy harvesting technology and low-power microcontrollers, tiny sensor devices can now be built that can be deployed indefinitely with little or no required maintenance. This rising generation of devices includes RPID-scale batteryless computers that rely entirely on RF or solar energy harvested from the environment, and can be deployed in many locations that are poorly suited to traditional battery-powered devices (e.g. embedded in permanent structures and building materials and implanted in living tissues). Batteryless devices can be made smaller, cheaper, lower maintenance, and more environmentally friendly than battery-based systems; however, they also face a variety of challenges. Often these devices store only enough energy for a few seconds of operation in a tiny capacitor. Tighter energy budgets mean increased power-supply volatility and power failures as the common case instead of a rare exception. In this environment, small changes in system behavior can easily make the difference between success and failure.
A small number of systems have been designed to improve the robustness of these RFID-scale devices: using checkpointing to extend computations across failures (Mementos), and better scheduling to improve the probability that a task will complete successfully. However, time remains a significant challenge. Many sensing tasks and security-related computations are useful, only if the device has a sense of time.
Powering a real-time-clock (RTC) on such light energy budgets is often not feasible, and frequent power failures of varying duration make approximate time-estimation difficult. Requiring an external device (e.g. a RFID tag reader) to act as a time source introduces security problems and may either require significant infrastructure or severely limit range and mobility.
Providing steady power to an RFID-scale device is often an unrealistic expectation; however, a device may be able to harvest enough power to supply a low-power real-time clock (RTC).
A device using a RTC may achieve greater accuracy, but it would also require either special switching circuitry to preserve sufficient energy for the clock. In either case, the benefits of a RTC come at the cost of increased complexity, cost, and power consumption—all of which are at a premium in these tiny sensors,
There is a need for methods and systems for providing time information in intermittently-powered devices that are batteryless and operate purely on harvested energy.